Mixtures of mixed triglycerides, and more particularly anhydrous milk fat (AMF), have proven utility as a food component when separated into fractions containing different populations of triglycerides. However, difficulty in processing milk fat and fractionating the triglycerides in commercial scale has restricted its use in the food industry in the United States. In Europe, where fat fractionation is carried out in a batch process, the practice is more common.
The properties of milk fat vary greatly depending upon the season, the region, the breed of dairy cow and even the animal feed. Since AMF is composed of an exceedingly large variety of fat molecules in the form of triglycerides, the properties of AMF are a normalized average of the characteristics of its individual components. A homogenous fraction of any one or a group of related triglycerides has different physical properties (e.g. different melting points, blending properties with baking or confectionery ingredients, and textural characteristics). The characteristics of milk fat and associated physical properties such as melting point have been studied, but cannot be precisely determined from one batch to the next. In general, ordinary AMF has a wide melting range which makes it unsuitable for many food applications.
Fractionation of milk fat is a process of separating the triglyceride components of milk fat on the basis of their melting points. As a result of the varying characteristics of the milk fat, the fractionation of milk fat is as much art as it is science because the many variables are difficult to control in conventional batch procedures.
Fractionation is accomplished by the selective crystallization of the triglyceride components of a mass of melted AMF. The energy for crystallization is equal to the difference between the melting point of the triglycerides and the actual solution temperature. Crystallization from the melt is characterized by the formation of a lattice structure as the triglycerides molecules undergo a phase transition from liquid to solid. Crystallization occurs in two stages. The first is nucleation where embryonic crystals referred to as nuclei are formed. The second stage is crystal growth which involves diffusion of the triglycerides into the growing crystal lattice structure.
Interestingly, it is found that the actual distribution of fatty acids contained in the various fractions obtained at high, medium, or low temperatures from a melt does not differ as much as might be expected. This appears to be explained by the degree of saturation and chain length of the fatty acids on the intact triglyceride. Fractions having a high temperature (40-50.degree. C.) melting profile generally have a total carbon chain length for the three glycerols (acyl carbon number) of C44 to C54, and those having a low temperature (&lt;20.degree. C.) melting profile generally have a total carbon (C) chain length from C26 to C42. The relation of the triglyceride composition of different fractions to the crystalline forms and polymorphisms and to various product applications is reviewed in E. Deffense, JAOCS, 70: 1193 (1993) which is hereby incorporated by reference.
Currently, most milk fat fractionation is performed using the Tirtiaux dry fractionation process This process essentially involves crystallization by cooling of a commercial batch of milk fat. Depending on capacity of the equipment from 10 to 100 tons/day of AMF may be processed. The crystallization is performed by heating the AMF to create a stabilized melt and then cooling and agitating the melt and collecting a solid fraction crystallizing at a relatively high temperature. The process is repeated to collect fractions crystallizing at successively lower temperatures.
In the prior art, it is believed that the agitation of milk fat during cooling improves the crystallization, thereby improving the fractionation. The batch of milk fat is slowly cooled and gently agitated in a slow, controlled process, taking up to 24 hours for a single batch. Nucleation and crystallization both occur in an agitated environment.
In the Tirtiaux method, the hard fractions obtained are collected over a range of upper temperatures to form hard stearin, and a liquid portion known as olein. In a repeat of the crystallization step on the olein fraction, a second soft stearin and olein fraction are obtained. This olein can then be further cooled to lower temperatures to create fractions with intermediate (20-30.degree. C.) and low (&lt;15.degree. C.) melting points. These fractions can be used in various compositions to yield milk fat/oil homogenates with improved properties in confectioneries and shortenings.